I've also found that if I copy something from another program (e.g. gnome-terminal), when I right-click on an input field in Steam, it immediately pastes whatever I copied before opening the context menu. However, using the paste option in the context menu does not paste anything that I copy from other programs.

For me, copy/paste works with the shortcuts. But its a little odd, I copy something, then go into steam, paste, but it pastes what I had copied previously. Then when I undo then paste it again, it pastes the most recent copy.

I tried to redeem some keys from the humble bundles i've bought and worked for me. i'm on Kubuntu 12.10. It's true that clicking in the window couldn't get the focus at first, but clicking again gain it and the paste part works.

I seem to be having a similar issue... sometimes when I Ctrl +v something into the chat window it will paste the correct thing, but other times it pastes the previous thing that was copied to the clipboard. Doesn't really seem to be a pattern to it. When you paste a second time, it pastes the correct item. Ubuntu 12.10 x64 here....